1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with a magnetic recording medium in which the output is increased over a wide wavelength region from long wavelengths to short wavelengths and in the case of subjecting the magnetic recording medium in the form of a disk to rotation and reproduction, in particular, the width of output fluctuation with its rotation period is reduced.
2. Description of the Prior Art
Up to the present time, magnetic recording media have widely been used for magnetic recording and reproducing, which are obtained by dispersing a ferromagnetic substance consisting of acicular crystals of .gamma.-Fe.sub.2 O.sub.3 or CrO.sub.2 in a binder and coating the resulting coating composition onto a non-magnetic support. Lately, however, improvement of the recording density has eagerly been desired so as to achieve the enlargement of a recording capacity and the miniaturization, but it is necessary to make the maximum size of an acicular magnetic grain sufficiently smaller than a recording wavelength or recording bit length in order to obtain a recording medium suitable for high density recording using an acicular magnetic powder as in the prior art. At the present time, an acicular magnetic powder with a size of about 0.3 .mu.m has been put to practical use to obtain a shortest recording wavelength of about 1 .mu.m.
For the purpose of obtaining a medium whereby a further high density recording is made possible in future, it will be required to make further small the size of an acicular magnetic powder. In such a small acicular magnetic powder, however, the thickness is extremely fine, e.g. 100 .ANG. or less and the grain volume is very small, e.g. 10.sup.-17 cm.sup.3 or less, which result in the problem that the magnetic properties are deteriorated by thermal agitation or surface effects and a sufficient orientation cannot be obtained even if a magnetic field is applied to the magnetic coating film.
In a magnetic recording medium coated with a coating composition in which an acicular magnetic substance having an acicular ratio (long axis/short axis) of more than 10 is dispersed, the direction of coating due to the acicular shape remains so that in the case of using the magnetic recording medium by rotating in the form of a disk, an output fluctuation with its rotation period occurs.
Accordingly, there have hitherto been proposed magnetic recording media each using, as a ferromagnetic substance, hexagonal ferrite which is plate-shaped and has the axis of easy magnetization in the vertical direction to the plate surface, and each being subjected to magnetic orientation in the longitudinal direction in the plane of the recording medium (Japanese Patent Application OPI (Kokai) Nos. 6525/1983 and 6526/1983). However, thee magnetic recording media using hexagonal ferrites have the drawback that the output is high at high frequency side but is low at low frequency side.